displaying the disk usage using df
and displaying the disk usage using du -sk
gives me different results:

1st case: df -k |grep users

2nd case: cd /users; du -sk

sometimes this difference is 50 gigabyte!!!
do you have any idea about this situation?

You are aware that 'df' only reports usage stats on filesystems, yes? 'du', on the other hand, measures directory contents.

disk storage is used for what else than directory contents?

Is there just one mount point that has the word "users" as part of the filename?
Check the output for my system:

# cd /netbackup
# df -k |grep netbackup
/dev/vx/dsk/sysdg/netbackup 1536000 1439224   96080    94%    /netbackup
# du -sk .
1422240 .

currently i cannot post actual output as we dont have this problem now.
we killed almost all the processes running on the machine,
to be able to make some links to another disks.
right after killing the processes we observed that,
output of df , rapidly decreased and came to a value very close to
output of du.
this may be a coincidence.
but another possibility is that the processes may allocate some temporary space
in the disks, because there were lots of long-time running processes which are doing heavy operations on database and also on unix files.
does it make sense?

i will send the actual outputs if the problem occurs again.

thanks for your concern...

currently we have 0.7 gigabyte difference:

root@london:/> cd /users
root@london:/users> du -sk
70384708 .
root@london:/users> df -k| grep /users
DGC051#fs2 108000000 71011701 36988299 66% /users
root@london:/users>

See this link - differences between du and df